The RNAV type surface navigation allows an aircraft to fly from one waypoint to another waypoint, rather than from ground stations (of radio-navigation means of the NAVAID type) to ground stations.
As known, the RNP concept corresponds to a surface navigation for which monitoring and warning means (on board the aircraft) are added, allowing to ensure that the aircraft remains in a corridor, referred to as RNP, around a reference path and authorizing curved paths to be taken into consideration. Outside this corridor there are potentially reliefs other aircrafts. The performance required for a RNP operation type is defined by a RNP value representing half the width (in nautical miles: NM) of the corridor around the reference path, wherein the aircraft should remain for 95% of the time during the operation. A second corridor (around the reference path) having half a width being twice the RNP value is also defined. The probability that the aircraft goes out of this second corridor should be lower than 10−7 per hour of flight.
The RNP AR operation concept is even more requiring. The RNP AR procedures are indeed characterized by:                RNP values:                    being lower than or equal to 0.3 NM in approach, and being able to go down to 0.1 NM; and            being strictly lower than 1 NM at the start and upon throttling up, and being also able to go down to 0.1 NM;                        a final approach segment that could be curved; and        obstacles (mountains, traffic . . . ) that could be located at twice the RNP value with respect to the reference path, while for the usual RNP operations, an additional margin with respect to obstacles is provided.        
The air authorities have defined a TLS (<<Target Level of Safety>>) of 10−7 per operation, whatever the type. In the case of the RNP AR operations, as the RNP values are able to go down to 0.1 NM and the obstacles could be located at twice the RNP value of the reference path, such an objective results in a probability that the aircraft goes out of the corridor with half a width D=2·RNP that should not exceed 10−7 per procedure.
The equipment on board aircrafts (flight management system, inertial unit, means for updating GPS data and means for guiding the autopilot), as well as the usual architecture, does not allow to reach the target level of safety, if mitigation operational means are not provided including for the detection and the management of possible breakdowns. This is why a special authorization is required for such a type of operation, so as to ensure that the operational procedures and the training of pilots allow to reach the target level of safety. Furthermore, as the crew should take charge in some breakdowns, today's aircrafts are not able to ensure a RNP value of 0.1 NM upon a breakdown, as the crew is not able to meet the performance requirements in manual piloting.
On current aircrafts, the monitoring of RNP AR operations is implemented by means of two usual functions, that is:                a first function monitoring the accuracy and the integrity of the position calculation:                    the accuracy of the position is compared to once the RNP value;            the integrity is compared to twice the RNP value; and            if one of the two parameters, either accuracy or integrity exceeds the allotted threshold, a warning is emitted and the crew should take appropriate actions; and                        a second function allowing the crew to monitor the guidance of the aircraft:                    the lateral and vertical deviations of the aircraft with respect to the reference path are displayed and shown to the crew;            the crew monitors the deviations with respect to the budgets allotted for each deviation. Should the crew detect an excessive deviation, it should re-manage the aircraft again and take adequate corrective actions.                        
As previously indicated, current aircrafts are not able to guarantee a RNP value of 0.1 NM upon a breakdown and the crew should be specially trained for following the RNP AR procedures. Indeed, the crew should be able to detect and adequately process breakdowns being able to compromise the on-going operation.
The objective for future aircrafts is to be able to follow the RNP AR procedures with RNP values up to 0.1 NM, and this without restriction (in normal situation and in case of a breakdown) on a start, approach and throttling up. To this end, the crew should no longer be considered as the main means for detecting and processing breakdowns.